In Ang Lee’s Lust, Caution, a painfully high-toned erotic drama set in Japanese-occupied Shanghai during World War II, Tony Leung, as a poshly repressed Chinese government minister, and Tang Wei, as a willowy student resistance fighter who seduces him as part of an assassination plot, spend a fair amount of time with their clothes off. (At least, they do once you’ve sat through an eternity of muffled espionage noodling.) It’s fair to say that most of what they do in their nakedness is easy on the eyes. The first time the two go at it, Leung, a veteran Hong Kong matinee idol who can look about as severe as Dracula, pulls off his belt and administers a beating to get her in the mood. Tang, a flashing-eyed newcomer wearing complicated hair and a conspiratorial bud of a smile, responds with gasps of pleasure. There are ripples of coercion and torment throughout their lovemaking — an ecstasy just this side of agony. Torsos glisten, limbs intertwine as if they belonged to double-jointed acrobats, and at one point there’s a shot of intercourse that, unless Lee is using some effects that George Lucas never thought of concocting, makes it look a lot like the two actors are truly doing it.

The real kink, however, is in the situation. Tang’s Wong Chia Chi is a student spy, part of a group of young radicals who want to bring down Leung’s high-level collaborator, but she is also turned on by his power, his domineering fascist-aristocratic entitlement — everything she claims to despise about him politically. She loves him, in part, because she hates him. As for Leung’s Mr. Yee, he has never tapped a desire this dark. For years, directors have dreamed of making a serious erotic drama with lyrically raunchy, even hardcore sex scenes. In films like 9 Songs and the nifty Shortbus, the dream has recently come true — and audiences have yawned. Lust, Caution is the latest NC-17 hothouse, and as long as the two actors are writhing, locked in a grip of sadomasochistic romance, the movie has an avid sensual glow.

The rest of the time (and that’s a lot of time — it’s 2 hours and 38 minutes long), Lust, Caution is like a ’40s Hollywood thriller made sedate and gorgeous and aesthetic and remote. Ang Lee, as he proved in Brokeback Mountain, can be a master at teasing out the emotion roiling under the surface of things, but Lust, Caution has more surface than emotion. Based on the Eileen Chang short story, the movie would have been better staged as fast, tawdry, voluptuous pulp, which is really all it is.

Lee makes the assassination plot maddeningly repetitious. There’s nothing too interesting about this student group — its leader, played by Asian pop star Wang Leehom, is a humorless fanatic — and the fact that Wong’s sympathies get tangled as they do is a complication Lee takes far too long to get to. At a restaurant, or a tailor’s shop, Leung and Tang work to make their tango of repression vivid, but the scenes are so static and freighted it’s as if Lee were fetishizing their reserve. Lust, Caution wants us to feel the erotic ping of buttoned-up people ripping open those buttons, but too often it’s the film’s drama that’s under wraps. C+